[While it didn't actually look like something was wrong with her now, not knowing what had just been done to her would've certainly been at least somewhat scary. At least Ai would have been, in her position, so Belle's reaction to the girl's question was not unexpected.]

...Maybe going through the guide in your journal would be a good idea, Belle-san. It mentions most of the possible experiments that the Malnosso can do to people here. You may find out that way.

[That way Belle may check whether she has the symptoms of a given experiment if she couldn't tell what had been done to her right away. At the very least, she apparently hadn't lost a lot of time away from Luceti. If Ai were to be abducted and experimented on, she wouldn't want to be gone for long either.]